Shearing interferometer with dynamic pupil fill

ABSTRACT

A wavefront measurement system includes a source of electromagnetic radiation. An illumination system delivers the electromagnetic radiation to an object plane. A source of a diffraction pattern is in the object plane. A projection optical system projects the diffraction pattern onto an image plane, which includes a mechanism (e.g., a shearing grating) to introduce the lateral shear. A detector is located optically conjugate with the pupil of the projection optical system, and receives an instant fringe pattern, resulting from the interference between sheared wavefronts, from the image plane. The diffraction pattern is dynamically scanned across a pupil of the projection optical system, and the resulting time-integrated interferogram obtained from the detector is used to measure the wavefront aberration across the entire pupil.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to photolithography systems,and more particularly, to measuring wavefront parameters in aphotolithographic system.

2. Related Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays, circuit boards, various integrated circuits, andthe like. A frequently used substrate for such applications is asemiconductor wafer. One skilled in the relevant art would recognizethat the description herein would also apply to other types ofsubstrates.

During lithography, a wafer, which is disposed on a wafer stage (WS), isexposed to an image projected onto the surface of the wafer by anexposure system located within a lithography system. The exposure systemincludes a reticle (also called a mask) for projecting the image ontothe wafer.

The reticle is usually mounted on a reticle stage (RS) and generallylocated between the wafer and a light source. In photolithography, thereticle is used as a photo mask for printing a circuit on the wafer, forexample. Lithography light shines through the mask and then through aseries of optical lenses that shrink the image. This small image is thenprojected onto the wafer. The process is similar to how a camera bendslight to form an image on film. The light plays an integral role in thelithographic process. For example, in the manufacture of microprocessors(also known as computer chips), the key to creating more powerfulmicroprocessors is the size of the light's wavelength. The shorter thewavelength, the more transistors can be formed on the wafer. A waferwith many transistors results in a more powerful, faster microprocessor.

As chip manufacturers have been able to use shorter wavelengths oflight, they have encountered a problem of the shorter wavelength lightbecoming absorbed by the glass lenses that are intended to focus thelight. Due to the absorption of the shorter wavelength light, the lightfails to reach the silicon wafer. As a result, no circuit pattern iscreated on the silicon wafer. In an attempt to overcome this problem,chip manufacturers developed a lithography process known as ExtremeUltraviolet Lithography (EUVL). In this process, a glass lens can bereplaced by a mirror.

The problem of measuring the undesirable perturbations of the wavefront(often referred to as wavefront aberrations) is a persistent one for thelithographic applications. These wavefront aberrations result fromvarious physical causes, such as changes in refractive or reflectiveproperties of the optical elements (lenses or mirrors) occurring as aresult of mechanical displacements or deformations, or changes in theoptical properties of the optical elements caused by heating, orlight-induced compaction. In particular, it is desirable to be able tomeasure wavefront quality in the photolithographic tool during waferproduction and exposure, rather than having to take the tool offline inorder to do so, which increases cost of ownership, reduces through-putor introduces some other type of inefficiency.

SUMMARY OF THE INVENTION

The present invention is directed to a scanning interferometer withdynamic pupil fill that substantially obviates one or more of theproblems and disadvantages of the related art.

An embodiment of the present invention includes a wavefront measurementsystem including a source of electromagnetic radiation. An illuminationsystem delivers the electromagnetic radiation to an object plane. Anobject generates a diffraction pattern and is located in the objectplane. A projection optical system projects an image of the object ontoan image plane. A detector receives a fringe pattern from the imageplane. The diffraction pattern is scanned across a pupil of theprojection optical system.

Another embodiment of the present invention includes a wavefrontmeasurement system with an illumination system that deliverselectromagnetic radiation at an object plane. A source of a beam of theelectromagnetic radiation is in the object plane. A projection opticalsystem focuses the beam onto an image plane. A detector receives afringe pattern of the beam from the image plane. The beam is scannedacross a pupil of the projection optical system.

Another embodiment of the present invention includes a method ofmeasuring a wavefront of an optical system including generatingelectromagnetic radiation at a source; delivering the electromagneticradiation at an object plane of the optical system; generating adiffraction pattern at the object plane; scanning the diffractionpattern across a pupil of the optical system; receiving an image of thesource while scanning the diffraction pattern; and determining wavefrontparameters from the image.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to illustrate exemplaryembodiments of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 shows a portion of an exemplary photolithographic system of thepresent invention.

FIGS. 2 and 3 illustrate the use of an interferometer to produce shearwavefronts.

FIG. 4 illustrates an example of interference fringes as they appear atthe focal plane with the use of the present invention.

FIG. 5 illustrates the rationale for the present invention, showing theoptical exposure system in a stylized, schematic form.

FIG. 6 shows how magnitudes of the diffraction orders and change in aninterferogram with an extended object in the object plane.

FIG. 7 illustrates an effect of modulating the extended object by aRonchi grating.

FIG. 8 is another illustration of the arrangement of the opticalelements that may be used in the present invention.

FIG. 9 is an illustration of dynamic pupil fill using a tiltingreflective Ronchi grating.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

It is convenient to characterize field-dependent aberrations of aprojections optics (PO) by an aberration of a wavefront of a sphericalwave emitted from a corresponding field point in the object plane.Various interferometry techniques can be used to measure aberration ofthis spherical wave. Shearing interferometry based on an extendedincoherent source in the object plane superimposed with an object-planegrating matching the shearing grating is described in J. Braat and A. J.E. M. Janssen, Improved Ronchi test with Extended Source, J. Opt. Soc.Am. A, Vol. 16, No. 1, pp. 131-140, January 1999, incorporated byreference herein. Also, the paper by Naulleau et al., Static MicrofieldPrinting at the ALS with the ETS-2 Set Optic, Proc. SPIE 4688, 64-71(2002) (http://goldberg.lbl.gov/papers/Naulleau_SPIE_(—)4688(2002).pdf),incorporated by reference herein describes the dynamic pupil fillillumination system for EUV implemented in order to control partialcoherence during printing at a synchrotron light source whereillumination is coherent.

FIG. 1 illustrates a photolithographic system 100 according to thepresent invention. The system 100 includes an illumination source 105, acondenser lens 102, an extended object 103 (located in the objectplane), projection optics 104 with a pupil 105, an image plane shearinggrating 106, a detector lens 107 and a CCD detector 108, arranged asshown in the figure. These elements will be discussed further below.

The grating 106 includes both transmissive and opaque regions. Theopaque regions can be formed of materials that absorb the radiation (forexample, for 13.5 nm exposure wavelength in the case of EUV lithography,or optical radiation in the case of lithographic systems using longerwavelengths), such as nickel, chromium or other metals.

It will also be appreciated that although the present invention isapplicable to lithographic systems that use refractive optical elements(such as the projection optics 104, and the imaging optics), theinvention is also applicable to systems using other wavelengths, withappropriate transmissive/refractive components used in place ofreflective ones, as needed.

The grating 106 also can include reflective (or opaque) regions. Thesereflective regions can be formed of materials that absorb the radiation(for example, for 13.5 nm EUV exposure wavelength), such as nickel,chromium or other metals.

The pitch of the grating 106 is chosen to provide an appropriate shearratio, where the CCD detector 108 is in the fringe plane (i.e., belowthe focal, or image, plane of the system), and “sees” a pattern offringes (an interferogram) or a number of overlapping circles, as willbe discussed further below. The shear ratio is a measure of the overlapof two circles, where a shear ratio of zero represents perfect overlap.Note also that it is desirable for the CCD detector 108 to “see” onlythe zeroth order and the + and −1^(st) order diffraction images, and toeliminate the + and −2^(nd) order diffraction images. Furthermore, theextended object 103 is constructed to aid in eliminating unwantedorders. It is important, however, that whichever pattern of transmissionand reflection areas is used, that it be a regular pattern.

The pitch of the source module grating 203 is also preferably chosen tomatch the pitch of the shearing grating so as to redistribute the lightin the pupil to those locations that will mutually overlap as a resultof shearing.

FIGS. 2 and 3 illustrate reference wavefronts and shear in a lateralshearing interferometer 210. The lateral shearing interferometer 210interferes a wavefront with itself, or, phrased another way, itinterferes a shifted copy of the wavefront with itself. As shown inFIGS. 2 and 3, the grating 106, positioned in the image plane, acts as ashearing interferometer, and generates a transmitted waves 204 with awavefront 211A, and a diffracted reference wave 205 with a wavefront211B. Thus, the lateral shearing interferometer 210 creates one or moreapparent sources, whose wavefronts 211A, 311B interfere to producefringes 212.

FIG. 4 illustrates the wavefront fringes (212A, 212B in FIG. 2) as seenby the CCD detector 108. As shown in FIG. 4, in the upper right-handphotograph, sheared fringes for a single object space slit are shown,where the slit is positioned in front of an incoherent, diffuse sourcethat fills the maximum numerical aperture and smoothes any wavefrontinhomogeneities. The bottom right-hand figure shows a fringe visibilityfunction 401, with zeroth order pattern 502 and first order diffractionpatterns 403. The 50% duty cycle on the grating 106 makes all evenorders of the diffraction pattern invisible. At the bottom left of FIG.4, the image space sharing grating 106 is shown, with a shear ratio of0.5.

In practice, a source point (101) is replaced by the “extended object103” to increase the optical throughput. The minimal dimensions of theextended object 103 are dictated by the available illumination power andthe sensitivity of the detector device used to measure theinterferograms. Depending on the object-side numerical aperture (NA) ofthe projection optics 104 and the optical throughput requirements, it isoften the case that the angular width of the diffraction pattern fromthe extended object 103 is small compared to the object-side NA of thePO 104. In that case, most of the light from this object ends upconcentrated within a small area of the PO 104 pupil 105. Even for thehighest pupil fills of the projection optics 104, the pupil 105 is stillnot completely filled, as is preferred for a complete aberrationmeasurement. In this situation, the wavefront measurement methods willhave very little sensitivity to the PO 104 aberrations occurring outsidea relatively small illuminated area of the pupil 105. It is thereforepreferred to fill the pupil 105 of the PO 104 more or less uniformly.

Thus, the problem of measuring wavefront aberrations has to balance twocompeting interests: filling the entire pupil 105 (but at the cost ofvery low intensity), or having sufficient intensity, but only on a smallportion of the pupil 105.

The following methods can be used to ensure the desired pupil fill inshearing interferometry:

-   -   (1) introducing a transmissive pattern inside the extended        object 103 that matches the shearing grating (e.g., a Ronchi        grating) and providing a fully incoherent illumination of the        extended object 103 [see Baselmans, supra]; and    -   (2) placing an extended incoherent source into an object plane        (by using a critical illumination or a diffuser in the object        plane) and superimposing it with a Ronchi grating that matches        the shearing grating [see Bratt et al., supra].

These methods have several problems:

-   -   (1) speckle disturbance of the measured interferograms occurs        due to the remaining coherence of the effective source (this        applies to methods 1 and 2) or due to a finite size of the        object plane diffuser elements (this applies to method 2). The        speckle disturbance adds high-frequency intensity fluctuations        to the measured interferograms, resulting in wavefront        measurement errors;    -   (2) the need to switch to a special illumination mode (this        applies to methods 1 and 2) during the measurement complicates        the wavefront measurement process; and    -   (3) a significant portion of light is diffracted from the        extended object 103 away from the pupil 105 and does not        participate in a formation of the sheared interferogram.

The present invention thus applies to the situation when the size of theextended object 103 needed to ensure the required optical throughput issuch that the characteristic width of the diffraction pattern is muchless than the object side NA of the PO 104, i.e., λ/extended objectsize<<NA object.

The pupil fill by the extended object 103 can be achieved dynamically.During the measurement of the interferogram, the extended object 103 canbe dynamically modified, so that the diffraction pattern from thisobject scans across the whole entrance pupil 105. The CCD detector 108that measures the sheared interferogram integrates (or sums) themomentary interferograms occurring in the process of measurement.

The dynamic modification of an extended object 103 can be performed byusing a reflective element, such as a tilting mirror, or by using arefractive object with varying slope and/or other characteristics (e.g.,a parabolic or spherical lens) moved against an aperture.

The reflected and transmissive extended objects described above usedynamic phase variation of light induced by linearly varying complexreflectivity/transmittance within or across the extended object 103.However, arbitrary (non-linear) phase variation effect can be also usedto fill the pupil 105 dynamically. Many physical arrangements well knownto those skilled in the art are possible for realizing such arbitrarynon-linear phase variations. For instance, they can be achieved by usingan extended movable object with its structure changing within or acrossit, and/or dynamically deformed, and/or otherwise dynamically modified(e.g., using spatial light modulators). Other possible realizations ofdynamically introduced phase variation can include a diffusor patternformed on a transmissive or reflective flexible substrate that can bephysically deformed, including plastics, piezoelectric materials, andstress-birefringent materials whose stresses are induced by actuators,etc.

Unless very small shears are utilized, in either of the above methods,the extended object 103 must have a transmittance pattern (an objectplane Ronchi grating matching the shearing grating) superimposed on itto provide additional redistribution of light in the pupil 105, asdescribed in Bratt et al. and Baselmans, cited above.

The dynamic modification is performed so that the transmittance functionwithin the extended object has a time-dependent linear variation of thephase that ensures that the diffraction pattern from the extended objectis shifted within the pupil 105, dynamically sweeping the pupil 105during the act of measurement.

The measurement of the interferogram is performed by the CCD detector108 that records energy distribution across the CCD detector 108 plane.The CCD detector 108 is capable of integrating the time-varyingintensity at every point in the detector 108 plane to collect asufficient number of photons during the act of measurement. The CCDarrays used in present-day wavefront sensors (like the CCD detector 108)satisfy this requirement.

As noted above, the dynamic modification of the extended object 103 canbe achieved by any number of mechanisms. For instance, a reflectiveextended object 103 may be used. Examples of such reflective objectsinclude:

-   -   (1) A tilting flat mirror can be used in a combination with an        aperture, or having only a small flat portion of a large tilting        object to be reflective. A relatively large tilting mirror is        easier to control (e.g., to tilt and rotate) compared to a        micro-mirror. The extended object 103 in this case coincides        with the tilting flat mirror, as shown in FIG. 9.    -   (2) A tilting micro-mirror, such as a mirror from a spatial        light modulator (SLM) array, can be used as the entire extended        object 103 (see FIG. 9). In order to sweep the pupil 105, the        micro-mirror has to tilt in two axes in the object plane. If the        micro-mirror can only tilt in one axis, it can be rotated around        the axis perpendicular to the object plane, thus allowing a        conical sweep of the 2D pupil 105. In practice, such a case is        rare.    -   (3) A reflective object with a varying slope of a reflective        surface, such as a parabolic or spherical mirror, which is moved        linearly behind a small aperture, can be used as the entire        extended object 103.

Note that in the case of reflective elements used to scan thediffraction pattern across the pupil 105, they need not always belocated at the object plane. For example, a flat tilting mirror could belocated between the object plane and the pupil 105 of the PO 104 (alsoacting to fold the optical axis of the system).

The extended object 103 can also be transmissive. In that case, thedynamic pupil fill can be achieved by moving a refractive element with avarying slope of one of its surfaces (e.g., a spherical or paraboliclens) against a small aperture, as shown in FIG. 8. A transmissivegrating can also be used, such that various regions on the grating havedifferent grating pitch, and the grating is moved linearly in its plane(i.e., perpendicular to the direction of the propagation of theelectromagnetic radiation) so as to vary the direction of the beam(i.e., to scan it across the pupil 105). It is also important to realizethat, depending on the particular type of extended object 103 used, thesize of the pupil 105 and the scanning approach, maintaining properfocus in the image plane may become a problem, as the diffractionpattern is being scanned across the pupil 105. However, it is currentlybelieved that although it is preferred to maintain focus, somede-focusing is acceptable.

Unless very small shears are utilized, in any of the aboveimplementations, the extended object 103 must have a transmittancepattern (an object plane Ronchi grating matching the shearing grating)superimposed on it to provide additional redistribution of light in thepupil 105, as described in Bratt et al. and Baselmans. In addition, anyof the above objects are preferably translatable in two lateraldimensions to accomplish the phase shift readout of fringes preferred inthe shearing interferometer measurement.

The final sheared interferogram measured by the CCD detector 108 is aresult of integration in time of the momentary sheared interferogramsresulting from most of the light concentrated within a small portion ofthe pupil 105. The momentary sheared interferograms may have highcontrast interference fringes only within a relatively small portion ofthe pupil image in the detector plane formed by the interferingdiffraction orders. Their time integral measured by the CCD detector 108has well-defined interference fringes across the whole pupil 105 thatcan be used (typically in conjunction with phase-stepping) to computethe wavefront aberration.

This is due to the fact that dynamic pupil fill described above isequivalent to the use of a stationary source corresponding to an actualsource convolved with the dynamic movement (source scanning). Thus,regardless of the degree of coherence of illumination from the actualsource, the effective source provides fully incoherent illumination.

FIG. 5 illustrates the rationale for the present invention, showing theoptical exposure system in a somewhat stylized, schematic form. Thisfigure relates to the use of a pinhole in the object plane in order togenerate the spherical wave that fills the pupil 105 and whoseaberration is measured by the shearing interferometer. As shown in FIG.5, going from top to bottom in the figure, light from a light sourcegoes through a condenser lens 102, and then through an object plane witha pinhole. The magnitude of the field at the pupil 105 of the projectionoptics 104 is shown by diagram A, where the pupil 105 coordinate isgiven as “f”. Light then is focused onto an image plane 605, then passesthrough optional detector projection optics 107, and is detected by thedetector 108 in the detector plane. Graph B shows magnitudes of the −1and +1 diffraction orders formed by the shearing grating, which islocated in the image plane 605. Graph C shows an interferogram resultingform the diffraction orders from the shearing grating. Note the visibleimage variation that is due to the aberrations (phase variations) thatare present in the resulting interferogram.

FIG. 6 shows how the magnitudes of the diffraction orders and change inthe interferogram, when an extended object 103 is placed in the objectplane that fills only a small portion of the pupil 105, resulting ininterferogram fringes of very small (negligible) contrast observed inthe detector plane within the non-overlapping peaks corresponding to thesheared diffraction pattern of the extended object 103. As in FIG. 5,Graph A in FIG. 6 shows the magnitude of the field in the pupil 105 ofthe projection optics 104, with the pupil coordinate “f”. Graph B showsthe magnitudes of the 0th, −1, and +1 diffraction orders formed by theshearing grating 106, when an extended object 103 is present in theobject plane. Graph C shows the interferograms resulting from thediffraction orders from the shearing grating. Because the diffractionorders do not overlap sufficiently, the resulting interferogram onlyweakly depends on the wavefront aberrations.

FIG. 7 illustrates the effect of modulating the extended object 103 by aRonchi grating, that redistributes the light in the pupil 105 to thoselocations that will mutually overlap as a result of shearing. As withFIGS. 5 and 6, Graph A shows the magnitude of the field in the pupil 105of the projection optics 104. Graph B shows magnitudes of the 0th, +1and −1 diffraction orders formed by the shearing grating, and Graph Cshows the resulting interferogram from the diffraction orders from theshearing grating and the matching Ronchi grating. As a result of theoverlap between the 0th, +1 and −1 diffraction orders, the interferogramin the overlap region (inside the peaks) strongly depends on thewavefront aberrations.

FIG. 8 is another illustration of the arrangement of the opticalelements that may be used in the present invention, namely a dynamicpupil fill using a moving refractive object. The illustration of FIG. 8is primarily applicable to a transmissive extended object 103. Forexample, as shown in FIG. 8, a transmissive Ronchi grating 801 can beused, with a refractive object having a varying slope that is movedagainst the object plane. The graphs on the lower right of FIG. 8illustrate the interferograms resulting with this arrangement. Note thatthe refractive object, as noted above, may be, for instance, a sphericalor a parabolic lens that is being moved against a small aperture.

FIG. 9 is another illustration of dynamic pupil fill using a tiltingreflective Ronchi grating 901, with the beam patterns not shown. Asshown in FIG. 9, a beam splitter 902 may also be necessary. Thereflective extended object 103 (in this case, the Ronchi grating 901) isplaced on a tilting mirror. The diagrams at bottom right illustrate theresulting interferogram patterns. The large tilting mirror can be usedin combination with an aperture, or can have only a small, flat partportion, or can be a small flat portion of a large tilting object, whichis made reflecting. A relatively large mirror, or a larger object, iseasier to handle (in other words, to tilt and rotate) compared to amicromirror. The extended object 103 in this case would coincide withthe large tilting mirror, as shown in FIG. 9.

Also, a tilting micromirror (such as a mirror in a spatiallight-modulator array) may be used as the entire extended object 103.

The present invention has a number of advantages over conventionalsystems. For example, the dynamic pupil fill eliminates the need for adiffuser in the object plane (e.g., in EUV wavefront sensors), thusresulting in elimination or reduction of speckle-induced wave frontmeasurement errors.

The dynamic pupil fill also eliminates the need to switch to a specialillumination mode during the wavefront measurement. The sameillumination mode used during the exposure can be used to perform thewavefront measurement. (However, one still has to properly position areticle stage with the tilting mirror on it.)

The dynamic pupil fill also allows to fill the PO pupil 105 “tightly,”thus significantly reducing the loss of light that occurs with othermethods. If necessary or desirable, the dynamic pupil fill allowssampling only the portions of the PO pupil that are of interest.

It will be understood by those skilled in the art that various changesin form and details may be made therein without departing from thespirit and scope of the invention as defined in the appended claims.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A wavefront measurement system comprising: a source ofelectromagnetic radiation; an illumination system that directs saidelectromagnetic radiation to an object plane; an object in the objectplane producing a diffraction pattern; a projection optical system thatprojects an image of said object onto an image plane; and a detectorthat receives a fringe pattern from said image plane, wherein thediffraction pattern is scanned across a pupil of said projection opticalsystem.
 2. The system of claim 1, wherein the object includes a tiltablemirror for scanning the diffraction pattern across the pupil.
 3. Thesystem of claim 1, wherein the object includes a diffraction gratingwith variable pitch for scanning the diffraction pattern across thepupil.
 4. The system of claim 1, wherein the object includes arefractive prism with a varying wedge angle for scanning the diffractionpattern across the pupil.
 5. The system of claim 1, wherein the objectincludes a spatial light modulator for scanning the diffraction patternacross the pupil.
 6. The system of claim 1, wherein the object generatesa non-linear phase variation across to scan the diffraction patternacross the pupil.
 7. The system of claim 1, wherein the object comprisesa stress-birefringent material to scan the diffraction pattern acrossthe pupil.
 8. The system of claim 1, further comprising reflectiveoptics for scanning the diffraction pattern across the pupil.
 9. Thesystem of claim 1, further comprising refractive optics for scanning thediffraction pattern across the pupil.
 10. The system of claim 1, whereinthe object includes a movable mirror with a variable surface slope forscanning the diffraction pattern across the pupil.
 11. The system ofclaim 1, wherein the diffraction pattern is dynamically scanned across apupil of said projection optical system.
 12. The system of claim 1,wherein the detector is located in a plane that is optically conjugatewith the pupil.
 13. The system of claim 1, further comprising a gratingin the image plane to generate the fringe pattern.
 14. A wavefrontmeasurement system comprising: an illumination system that deliverselectromagnetic radiation to an object plane; an object in the objectplane that generates a beam of the electromagnetic radiation; aprojection optical system that projects the beam onto an image plane;and a detector that receives a fringe pattern of the beam from the imageplane, wherein the beam is scanned across a pupil of the projectionoptical system.
 15. The system of claim 14, wherein the object includesa tiltable mirror for scanning the beam across the pupil.
 16. The systemof claim 14, wherein the object includes a diffraction grating withvariable pitch for scanning the beam across the pupil.
 17. The system ofclaim 14, wherein the object includes a refractive prism with a varyingwedge for scanning the beam across the pupil.
 18. The system of claim14, wherein the object includes a spatial light modulator for scanningthe beam across the pupil.
 19. The system of claim 14, wherein thesource of the diffraction pattern generates a non-linear phase variationacross to scan the diffraction pattern across the pupil.
 20. The systemof claim 14, wherein the source of the diffraction pattern includes astress-birefringent material for scanning the beam across the pupil. 21.The system of claim 14, further comprising reflective optics forscanning the beam across the pupil.
 21. The system of claim 14, furthercomprising refractive optics for scanning the beam across the pupil. 22.The system of claim 14, wherein the object includes a movable mirrorwith a variable surface slope for scanning the beam across the pupil.23. The system of claim 14, wherein the detector is located in a planethat is optically conjugate with the pupil.
 24. The system of claim 14,further comprising a grating in the image plane to generate the fringepattern.
 25. A method of measuring a wavefront of an optical systemcomprising: generating electromagnetic radiation at a source; deliveringsaid electromagnetic radiation to an object plane of said opticalsystem; generating a diffraction pattern at said object plane; scanningthe diffraction pattern across a pupil of said optical system; receivingan image of said source while scanning the diffraction pattern; anddetermining wavefront parameters from said image.
 26. The method ofclaim 25, wherein the scanning step includes tilting a mirror to directthe diffraction pattern across the pupil.
 27. The method of claim 25,wherein the scanning step includes moving a diffraction grating withvariable pitch to direct the diffraction pattern across the pupil. 28.The method of claim 25, wherein the scanning step includes adjusting arefractive prism with a varying wedge angle to direct the diffractionpattern across the pupil.
 29. The method of claim 25, wherein thescanning step includes adjusting a spatial light modulator to direct thediffraction pattern across the pupil.
 30. The method of claim 25,wherein the scanning step uses reflective optics to scan the diffractionpattern across the pupil.
 31. The method of claim 25, wherein thescanning step uses refractive optics to scan the diffraction patternacross the pupil.
 32. A method of measuring a wavefront of a projectionoptical system comprising: (1) delivering electromagnetic radiation atan object plane of the projection optical system so as to generate abeam directed at the projection optical system; (2) positioning adetector below an image plane of the projection optical system; (3)receiving a fringe pattern of the beam at the detector whilesimultaneously scanning the beam across a pupil of the optical system;and (4) calculating wavefront aberrations from the fringe pattern.
 33. Awavefront measurement system comprising: means for generatingelectromagnetic radiation; means for delivering said electromagneticradiation to an object plane of a projection optical system; means forgenerating a diffraction pattern in the object plane; the projectionoptical system that projects an image of said diffraction pattern ontoan image plane; and means for detecting a fringe pattern from said imageplane, means for scanning the diffraction pattern across a pupil of saidprojection optical system.